In recent years, most wireless communication systems have come to employ digitalized, high-efficiency transmissions. When a linear modulation system is applied to wireless communications, technology for curbing nonlinear distortion by linearizing amplification characteristics of a power amplifier and reducing adjacent-channel leakage power is important, especially on a transmitting side. Further, when using an amplifier with poor linearity and attempting to enhance power efficiency, technology for compensating for the resulting nonlinear distortion is essential.
To compensate for such nonlinear distortion, a distortion compensation apparatus, which applies predistortion (a distortion compensation value) corresponding to the nonlinear distortion generated by the power amplifier to a signal on the input side is known (for example, Patent Document 1: Japanese Patent Laid-open No. 2001-189685, Patent Document 2: PCT International Publication No. WO2003-103163, and Patent Document 3: Japanese Patent Laid-open No. 2005-101908).
This distortion compensation apparatus sets a distortion compensation value corresponding to a signal level to be amplified in a distortion compensation table beforehand, reads a corresponding distortion compensation value from the distortion compensation table in accordance with the input signal level, and applies this value to the input signal.
In a mobile device for a TDMA system, the uplink signal here constitutes a burst mode. Thus, the temperature of the mobile device power amplifier drops in a transmission signal stop state, and the temperature rises once again at the leading edge of a burst.
Then, the respective parameters of the power amplifier (power amplifier AM/AM: output amplitude fluctuation relative to an input amplitude, AM/PM: output phase fluctuation relative to an input amplitude, carrier leakage in orthogonal modulator, DC offset in orthogonal demodulator, and so forth) fluctuate as the result of a temperature fluctuation, giving rise to nonlinear distortion. Furthermore, the temperature rise and temperature drop in the above-mentioned power amplifier have characteristics, which are manifested by temporal delays in input signal changes due to the cumulative effects of heat. The relationship between the characteristics of temperature fluctuations like this and power amplifier output characteristics is known as the memory effect.
FIG. 1 is a diagram showing another example of the memory effect in such an amplifier. The horizontal axis represents time, and the vertical axis shows the relationship between input power (amplitude) A and the junction temperature K of an amplifying semiconductor power device. For example, when a large input power (amplitude) input in the past transitions until it reaches the smaller present input power (amplitude) A at time T1 (solid line), the fluctuation in temperature (broken line) lags behind the change in input power due to the accumulation of heat, and is the state of temperature K1 for the most part. By contrast, at time T2, the small input power transitions to rise up to the present input power A, but the fluctuation in temperature (broken line) is a low temperature K2 state.
That is, the temperatures K1, K2 differ at times T1, T2 even though the amplitude level is the same. This shows that the temperature does not ordinarily coincide with the heating value (proportional to power consumption) corresponding to an amplitude level due to the build up of heat in the metal surrounding the power device. This is referred to as the thermal memory effect.
Because the temperature accumulation (memory) at the time of the past input power remains like this, setting a (linearized) distortion compensation value corresponding to an input power (amplitude) value in the distortion compensation table does not enable adequate compensation for distortion. Consequently, if temperature fluctuations are taken into account, it becomes impossible to use a compensation table in which the same compensation value is set for the same input amplitude, requiring a different compensation method from that used when there is no memory effect.
That is, based on the relationship shown in FIG. 1, the distortion compensation values set in the distortion compensation table at different times T1, T2 for the same amplitude A can be the same when the memory effect is not a factor. Meanwhile, the junction temperature of a semiconductor amplifying device at time T1 can be expected to be higher than at time T2. Therefore, the AM/AM and AM/PM characteristics of an amplifier, which makes use of a power device having characteristics for which temperatures will differ in conformance with these times, will change in accordance with the temperature as shown in FIG. 2. In FIG. 2, A represents the AM/AM characteristic, and B represents the AM/PM characteristic. The AM/AM characteristic A is characterized in that gain diminishes in the area where the input level increases, and displaces in the direction of arrow a when the temperature rises. Conversely, the AM/PM characteristic is characterized in that phase rotation increases when the input level increases, and furthermore, displaces in the direction of arrow b in which the phase increases even more when the temperature rises.